Reducing Fin Wriggling in Fin-Thinning Process

ABSTRACT

A method includes depositing a silicon layer over a semiconductor region, forming dielectric isolation regions extending into the silicon layer and the semiconductor region, and recessing the dielectric isolation regions. A first portion of the silicon layer and a second portion of the semiconductor region are between the dielectric isolation regions, and protrude higher than top surfaces of the dielectric isolation regions to form a semiconductor fin. The semiconductor fin is thinned, and after the first semiconductor fin is thinned, the first portion of the silicon layer remains. A gate stack is formed on the semiconductor fin.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No. 63/379,939, entitled “Reducing Fin Wriggling in Fin-Thinning Process,” filed Oct. 18, 2022, and this application is also a continuation-in-part application of U.S. patent application Ser. No. 17/452,178, entitled “Controlling Fin-Thinning Through Feedback,” filed on Oct. 25, 2021, which is a continuation application of U.S. patent application Ser. No. 16/527,346, entitled “Controlling Fin-Thinning Through Feedback,” filed on Jul. 31, 2019, now U.S. Pat. No. 11,158,726, issued Oct. 26, 2021, which applications are incorporated herein by reference.

BACKGROUND

With the increasing down-scaling of integrated circuits and increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFETs) were thus developed. In conventional FinFET formation processes, the semiconductor fins may be formed by etching a silicon substrate to form trenches, filling the trenches with a dielectric material(s) to form Shallow Trench Isolation (STI) regions, and then recessing the STI regions. The silicon substrate portions between the recessed portions of the STI regions thus form semiconductor fins, on which the FinFETs are formed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1 through 10 illustrate the cross-sectional views and perspective views of intermediate stages in the formation of Fin Field-Effect Transistors (FinFETs) with thinned fins in accordance with some embodiments.

FIGS. 11 through 13 illustrate the processes for thinning semiconductor fins in accordance with some embodiments.

FIGS. 14 and 15 are flow charts for thinning semiconductor fins in accordance with some embodiments.

FIG. 16 illustrates the chemical structure of amine derivatives used in a fin-thinning process in accordance with some embodiments.

FIG. 17 illustrates the etching of semiconductor fins through spraying an etching solution on a wafer in accordance with some embodiments.

FIG. 18 illustrates a process flow for forming FinFETs and thinning fins in accordance with some embodiments.

FIGS. 19-21, 22, 23A, 23B, 24, 25A, 25B, 26-29, 30, 31A, and 31B illustrate the cross-sectional views and perspective views of intermediate stages in a fin-thinning process and the formation of FinFETs comprising thinned fins in accordance with some embodiments.

FIG. 32 illustrates a thinned silicon germanium fin when no semiconductor buffer layer is over the silicon germanium fin during the thinning process in accordance with some embodiments.

FIG. 33 illustrates a thinned silicon germanium fin when a semiconductor buffer layer is over the silicon germanium fin during the thinning process in accordance with some embodiments.

FIG. 34 illustrates a process flow for forming FinFETs and thinning fins in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

A method of forming Fin Field-Effect Transistors (FinFET) and the corresponding thinning processes of semiconductor fins are provided in accordance with some embodiments. Some variations of some embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. In accordance with some embodiments of the present disclosure, semiconductor fins for an n-type FinFET and a p-type FinFET are formed to achieve target widths. The formation processes of the semiconductor fins include etching isolation regions to form semiconductor fins, measuring the widths of the semiconductor fins, comparing the measured widths of the semiconductor fins with the target widths, generating an etching recipe based on the measured widths and the target widths, and using the etching recipe to thin the semiconductor fins. After the thinning, the thinned semiconductor fins may be re-measured. Based on the re-measurement results, re-work may be performed to thin the semiconductor fins again.

Furthermore, when a semiconductor fin, which may be formed of silicon germanium, is thinned, a semiconductor buffer layer may be formed and left on top of the thinned semiconductor fin. The semiconductor buffer layer applies a strain on the underlying semiconductor fin, so that the wriggling (bending) of the thinned semiconductor fin is reduced.

Embodiments will be described with respect to a specific context, namely the process of thinning semiconductor fins and forming corresponding FinFETs based on the thinned semiconductor fins. The concept of the discussed embodiments may also be applied to the structure and the processing of other structures having fins, which include, and are not limited to, the thinning of the channel regions of Gate-All-Around (GAA) nanowire transistors, nanosheet transistors, etc. Embodiments discussed herein are to provide examples to enable making or using the subject matter of this disclosure, and a person having ordinary skill in the art will readily understand modifications that can be made while remaining within contemplated scopes of different embodiments. Although method embodiments may be discussed as being performed in a particular order, other method embodiments may be performed in any logical order.

FIGS. 1 through 10 illustrate the cross-sectional views of intermediate stages in the formation of a first FinFET and a second FinFET in device regions 100 and 200, respectively of wafer 10 in accordance with some embodiments of the present disclosure. The corresponding processes are also reflected schematically in the process flow shown in FIG. 18 . In accordance with some embodiments, the device region 100 is an n-type FinFET region, in which an n-type FinFET is to be formed, and device region 200 is a p-type FinFET region, in which a p-type FinFET is to be formed. In accordance with other embodiments, device region 100 and 200 may be both n-type FinFET regions, both p-type FinFET regions, or a p-type FinFET region and an n-type FinFET region, respectively.

Referring to FIG. 1 , wafer 10 is formed. The respective process is illustrated as process 402 in the process flow 400 shown in FIG. 18 . Wafer 10 may include (bulk) substrate 20, which may be formed of silicon or other semiconductor materials. Substrate 20 may also be a Silicon-on-Isolation (SOI) substrate that includes a bottom semiconductor layer, an isolation layer (for example, formed of silicon oxide) over the bottom semiconductor layer, and a top semiconductor layer over the isolation layer. A p-well region (not shown) may be formed in device region 100. Device region 200 may include substrate 20 and an epitaxy semiconductor layer 21 over substrate 20. Substrate 20 and epitaxy semiconductor layer 21 may be collectively referred to as semiconductor substrate 20′. Also, substrate 20 in device region 100 and substrate 20′ in device region 200 are considered as portions of a substrate that extend into device regions 100 and 200. In accordance with some embodiments of the present disclosure, epitaxy semiconductor layer 21 includes germanium, and may include silicon germanium, or may include germanium without silicon. The germanium percentage in epitaxy semiconductor layer 21 may be higher than 30 percent, and may be in the range between about 30 percent and about 100 percent. An n-well region (not shown) may be formed in semiconductor substrate 20′. The n-well region may or may not extend below epitaxy semiconductor layer 21.

Referring to FIG. 2 , wafer 10 is etched to form trenches 23 and semiconductor strips 126 and 226. The respective process is illustrated as process 404 in the process flow 400 shown in FIG. 18 . In device region 100, the portions of substrate 20 between neighboring trenches 23 are referred to as semiconductor strips 126. In device region 200, the portions of substrate 20′ between neighboring trenches 23 are referred to as semiconductor strips 226. To form trenches 23, pad dielectric layer 28 and hard mask layer 29 are first formed as blanket layers on wafer 10. Pad dielectric layer 28 may be a thin film formed of silicon oxide. In accordance with some embodiments of the present disclosure, pad dielectric layer 28 is formed through deposition, for example, using Plasma Enhanced Chemical Vapor Deposition (PECVD). Pad dielectric layer 28 may act as an etch stop layer for etching hard mask layer 29. In accordance with some embodiments of the present disclosure, hard mask layer 29 is formed of silicon nitride, for example, using Low-Pressure Chemical Vapor Deposition (LPCVD), Atomic Layer Deposition (ALD), Plasma-Enhanced Chemical Vapor Deposition (PECVD), or the like.

A photo resist (not shown) is formed on hard mask layer 29 and is then patterned through a photo lithography process. Hard mask layer 29 is then patterned using the patterned photo resist as an etching mask to form hard masks 29 as shown in FIG. 2 . Next, the patterned hard mask layer 29 is used as an etching mask to etch pad dielectric layer 28 and the underlying substrate 20 and epitaxy semiconductor layer 21. The resulting structure is shown in FIG. 2 . The resulting trench 23 may penetrate through epitaxy semiconductor layer 21 to extend into the underlying semiconductor substrate 20. In device region 100, the portions of semiconductor substrate 20 between trenches 23 are referred to as semiconductor strips 126. In device region 200, the portions of semiconductor substrate 20 and epitaxy semiconductor layer 21 between trenches 23 are referred to as semiconductor strips 226.

In accordance with some embodiments, after the formation of semiconductor strips 126 and 226, a fin-thinning process may be performed to thin the semiconductor strips 126 and 226 in order to improve the gate control of the resulting FinFETs and to reduce the fin-width variation. The respective process is illustrated as process 406 in the process flow 400 shown in FIG. 18 . The details of the fin-thinning process are discussed in subsequent paragraphs referring to FIGS. 11 through 13 . In accordance with other embodiments, the fin-thinning process is not performed at this stage. Rather, the fin-thinning process may be performed after the formation of protruding semiconductor fins 126′ and 226′ as shown FIG. 4 , or after the removal of dummy gate stacks as shown in FIG. 9 .

Referring to FIG. 3 , isolation regions 22 are formed, which are alternatively referred to as Shallow Trench Isolation (STI) regions hereinafter. The respective process is illustrated as process 408 in the process flow 400 shown in FIG. 18 . STI regions 22 are formed by filling trenches 23 (FIG. 2 ) with a dielectric material(s), followed by a planarization process such as a Chemical Mechanical Polish (CMP) process or a mechanical grinding process to remove excessing portions of the dielectric material. The remaining portions of the dielectric materials(s) are STI regions 22. STI regions 22 may include a liner dielectric (not shown), which may be a thermal oxide layer formed through the thermal oxidation of a surface layer of the semiconductor materials. The liner dielectric may also be a deposited silicon oxide layer, silicon nitride layer, or the like formed using, for example, ALD, High-Density Plasma Chemical Vapor Deposition (HDPCVD), or Chemical Vapor Deposition (CVD). STI regions 22 may also include a dielectric material over the liner oxide, wherein the dielectric material may be formed using Flowable Chemical Vapor Deposition (FCVD), spin-on coating, or the like. The dielectric material over the liner dielectric may include silicon oxide in accordance with some embodiments. Due to the planarization process, the top surfaces of hard masks 29 and the top surfaces of STI regions 22 may be substantially level with each other.

Semiconductor strips 126 and 226 are between STI regions 22. It is appreciated that the structure difference in semiconductor strips 126 and 226 is an example to show that different materials may be used in device regions 100 and 200. In accordance with some embodiments, each of semiconductor strips 126 and 226 may include a single semiconductor layer formed of a same semiconductor material, or may include a plurality of semiconductor layers formed of different materials. These materials may include silicon; germanium; a compound semiconductor including carbon-doped silicon, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GalnAs, GaInP, and/or GaInAsP; combinations thereof, and/or multi-layers thereof.

Referring to FIG. 4 , STI regions 22 are recessed, so that the top portions of semiconductor strips 126 and 226 protrude higher than the top surfaces 22A of the remaining portions of STI regions 22 to form protruding semiconductor fins 126′ and 226′, respectively. The respective process is illustrated as process 410 in the process flow 400 shown in FIG. 18 . The portions of the semiconductor strips 126 and 226 lower than the top surfaces 22A remain to be referred to as semiconductor strips 126 and 226, respectively. The etching may be performed using a dry etching process, wherein HF₃ and NH₃, for example, are used as the etching gases. During the etching process, plasma may be generated. In accordance with alternative embodiments of the present disclosure, the recessing of STI regions 22 is performed using a wet etching process. The etching chemical may include an HF solution, for example.

In above-illustrated embodiments, the fins may be patterned by any suitable method. For example, the fins may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers, or mandrels, may then be used to pattern the fins.

In accordance with some embodiments, after the recessing for STI regions 22 to form protruding semiconductor fins 126′ and 226′, a fin-thinning process may be performed to thin the protruding semiconductor fins 126′ and/or 226′ to improve the gate control, and to reduce the fin-width variation. The respective process is illustrated as process 412 in the process flow 400 shown in FIG. 18 . The details of the fin-thinning process are discussed in subsequent paragraphs referring to FIGS. 11 through 13 . In accordance with other embodiments, the fin-thinning process is not performed at this stage. Rather, the fin-thinning process may be performed after the formation of trenches 23 (FIG. 2 ) but before the formation of STI regions 22 (FIG. 3 ), or after the removal of dummy gate stacks as shown in FIG. 9 .

In accordance with some embodiments, silicon caps (not shown) may be epitaxially grown on protruding semiconductor fins 126′ and 226′. Referring to FIG. 5 , dummy gate stacks 30 are formed to extend on the top surfaces and the sidewalls of (protruding) fins 126′ and 226′. The respective process is illustrated as process 414 in the process flow 400 shown in FIG. 18 . Dummy gate stacks 30 may include dummy gate dielectrics 32 and dummy gate electrodes 34 over dummy gate dielectrics 32. Dummy gate electrodes 34 may be formed, for example, using polysilicon, and other materials may also be used. Each of dummy gate stacks 30 may also include one (or a plurality of) hard mask layer 36 over dummy gate electrodes 34. Hard mask layers 36 may be formed of silicon nitride, silicon oxide, silicon carbo-nitride, or multi-layers thereof. Dummy gate stacks 30 may cross over a single one or a plurality of protruding semiconductor fins 126′ and 226′ and/or STI regions 22. Dummy gate stacks 30 also have lengthwise directions perpendicular to the lengthwise directions of protruding semiconductor fins 126′ and 226′.

Next, gate spacers 38 are formed on the sidewalls of dummy gate stacks 30. The respective process is illustrated as process 414 in the process flow 400 shown in FIG. 18 . In accordance with some embodiments of the present disclosure, gate spacers 38 are formed of a dielectric material(s) such as silicon nitride, silicon carbo-nitride, or the like, and may have a single-layer structure or a multi-layer structure including a plurality of dielectric layers.

Etching processes may be performed to etch the portions of protruding semiconductor fins 126′ and 226′ that are not covered by dummy gate stacks 30 and gate spacers 38, resulting in the structure shown in FIG. 6 . The respective process is illustrated as process 416 in the process flow 400 shown in FIG. 18 . The recessing may be anisotropic, and hence the portions of protruding semiconductor fins 126′ and 226′ directly underlying dummy gate stacks 30 and gate spacers 38 are protected, and are not etched. The top surfaces of the recessed semiconductor fins/strips 126/126′ and 226/226′ may be lower than the top surfaces 22A of STI regions 22 in accordance with some embodiments. The spaces left by the etched portions of semiconductor fins/strips 126/126′ and 226/226′ are referred to as recesses 40, which comprise the portions located on the opposite sides of dummy gate stacks 30, and the portions between remaining portions of protruding semiconductor fins 126′ and 226′.

Next, epitaxy regions (source/drain regions) 142 and 242 are formed by selectively growing (through epitaxy) semiconductor materials in recesses 40, resulting in the structure in FIG. 7 . Source/drain region(s) may refer to a source or a drain, individually or collectively dependent upon the context. The respective process is illustrated as process 418 in the process flow 400 shown in FIG. 18 . Depending on whether the resulting FinFET is a p-type FinFET or an n-type FinFET, a p-type or an n-type impurity may be in-situ doped with the proceeding of the epitaxy. For example, in device region 100, silicon phosphorous (SiP), silicon carbon phosphorous (SiCP), or the like may be grown. In device region 200, silicon germanium boron (SiGeB), silicon boron (SiB), or the like may be grown. In accordance with alternative embodiments of the present disclosure, epitaxy regions 142 and 242 comprise other types of semiconductor materials, for example, III-V compound semiconductors such as GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinations thereof, or multi-layers thereof.

After the epitaxy process, epitaxy regions 142 and 242 may be further implanted with an n-type impurity and a p-type impurity, respectively, to form source and drain regions, which are also denoted using reference numerals 142 and 242, respectively. In accordance with alternative embodiments of the present disclosure, the implantation process is skipped when epitaxy regions 142 and 242 are in-situ doped with the n-type and the p-type impurities during the epitaxy.

FIG. 8 illustrates a perspective view of the structure after the formation of Contact Etch Stop Layer (CESL) 46 and Inter-Layer Dielectric (ILD) 48. The respective process is illustrated as process 420 in the process flow 400 shown in FIG. 18 . CESL 46 may be formed of silicon oxide, silicon nitride, silicon carbo-nitride, or the like, and may be formed using CVD, ALD, or the like. ILD 48 may include a dielectric material formed using, for example, FCVD, spin-on coating, CVD, or another deposition method. ILD 48 may be formed of an oxygen-containing dielectric material, which may be a silicon-oxide based material such as silicon oxide, Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), or the like. A planarization process such as a CMP process or a mechanical grinding process may be performed to level the top surfaces of ILD 48, dummy gate stacks 30, and gate spacers 38 with each other.

In a subsequent process, the dummy gate stacks 30 including hard mask layers 36, dummy gate electrodes 34 and dummy gate dielectrics 32 are etched, forming trenches 52 between gate spacers 38, as shown in FIG. 9 . The respective process is illustrated as process 422 in the process flow 400 shown in FIG. 18 . The top surfaces and the sidewalls of protruding semiconductor fins 126′ and 226′ are exposed to trenches 52.

In accordance with some embodiments, after the removal of dummy gate stacks 30, a fin-thinning process is performed to thin the protruding semiconductor fins 126′ and/or 226′ to improve the gate control and to reduce the fin-width variation. The respective process is illustrated as process 424 in the process flow 400 shown in FIG. 18 . The details of the fin-thinning process are discussed in subsequent paragraphs referring to FIGS. 11 through 13 . In accordance with other embodiments, the fin-thinning process is not performed at this stage. Rather, the fin-thinning process may be performed after the formation of trenches 23 (FIG. 2 ) but before the formation of STI regions 22 (FIG. 3 ), or after the recessing of STI regions 22 as shown in FIG. 4 .

Next, as shown in FIGS. 10 , replacement gate stacks 160 and 260 are formed in trenches 52 (FIG. 9 ). The respective process is illustrated as process 426 in the process flow 400 shown in FIG. 18 . Gate stacks 160 include gate dielectrics 156 and gate electrodes 158, and gate stacks 260 include gate dielectrics 256 and gate electrodes 258. In accordance with some embodiments of the present disclosure, each of gate dielectrics 156 and 256 includes an Interfacial Layer (IL) as its lower part. The IL is formed on the exposed surfaces of the protruding semiconductor fins 126′ and 226′. The IL may include an oxide layer such as a silicon oxide layer, which is formed through the thermal oxidation of protruding semiconductor fins 126′ and 226′, a chemical oxidation process, or a deposition process. Gate dielectrics 156 and 256 may also include high-k dielectric layer(s) formed over the corresponding ILs. The high-k dielectric layer includes a high-k dielectric material such as hafnium oxide, lanthanum oxide, aluminum oxide, zirconium oxide, or the like. The dielectric constant (k-value) of the high-k dielectric material is higher than 3.9, and may be higher than about 7.0, and sometimes as high as 21.0 or higher. The high-k dielectric layer is formed as a conformal layer, and extends on the sidewalls of protruding semiconductor fins 126′ and 226′ and the sidewalls of gate spacers 38. In accordance with some embodiments of the present disclosure, the high-k dielectric layer is formed using ALD, CVD, PECVD, Molecular-Beam Deposition (MBD), or the like.

Further referring to FIG. 10 , gate electrodes 158 and 258 are formed on gate dielectrics 156 and 256, respectively. Each of gate electrodes 158 and 258 may include a plurality of stacked metal layers, which may be formed as conformal layers, and a filling-metal region filling the rest of the respective trench 52. The stacked metal layers may include a barrier layer, a work function layer over the barrier layer, and one or a plurality of metal capping layers over the work function layer.

FIG. 10 also illustrates the formation of hard masks 162 and 262 in accordance with some embodiments. The respective process is illustrated as process 428 in the process flow 400 shown in FIG. 18 . The formation of hard masks 162 and 262 may include performing an etching process to recess gate stacks 160 and 260, so that recesses are formed between the opposite portions of gate spacers 38, filling the recesses with a dielectric material, and then performing a planarization process such as a CMP process or a mechanical grinding process to remove excess portions of the dielectric material. Hard masks 162 and 262 may be formed of silicon nitride, silicon oxynitride, silicon oxy-carbo-nitride, or the like. FinFETs 164 and 264 are thus formed in device regions 100 and 200, respectively. In subsequent processes, source/drain silicide regions, source/drain contact plugs, gate contact plugs, and the like, are formed.

FIGS. 11 through 13 illustrate the cross-sectional views in the fin-thinning processes in accordance with some embodiments of the present disclosure. The fin-thinning processes may be performed in various stages. For example, the fin-thinning processes may be performed after the formation of trenches 23 as shown in FIG. 2 , after the recessing of STI regions 22 as shown in FIG. 4 , or after the removal of the dummy gate stacks to form trenches 52 (FIG. 9 ). During these stages, the sidewalls of the protruding semiconductor fins 126′ and 226′ or the semiconductor strips 126 and 226 are exposed, allowing the fin-thinning process to be performed. The cross-sectional views of the structures in device region 100 (FIGS. 11 through 13 ) may be obtained from the reference cross-section A-A in FIG. 4 or FIG. 9 , depending on when the fin-thinning processes are performed. The cross-sectional views of the structures in device region 200 may be obtained from the reference cross-section B-B in FIG. 4 or FIG. 9 , depending on when the fin-thinning processes are performed. The fin-thinning processes performed on the structure shown in FIG. 2 may also be realized by applying the processes shown in FIGS. 11 through 13 . The resulting structure is similar to the structure shown in FIG. 2 , with the semiconductor strips and the corresponding silicon portions and silicon germanium (or germanium) portions becoming narrower as a result of the fin-thinning.

FIG. 14 illustrates a process flow 300 of the fin-thinning process in accordance with some embodiments, which process flow is discussed referring to FIGS. 11 through 13 . In process 302 as in FIG. 14 , protruding semiconductor fins 126′ and 226′ are formed to protrude higher than the top surfaces of the adjacent STI regions 22, which process also corresponds to the processes shown in FIG. 4 and FIG. 9 . The resulting cross-sectional views are shown in FIG. 11 . Also, in process flow 300, the first-type fins (the fins for forming first-type FinFETs) and the second-type fins (the fins for forming second-type FinFETs) are of opposite conductivity types, and the first-type fins may be either n-type fins or p-type fins, and the second-type fins may be either p-type fins or n-type fins. In the following discussed examples, it is assumed that protruding semiconductor fins 126′ (first-type fins) are formed of silicon, and the protruding semiconductor fins 226′ (second-type fins) are formed of silicon germanium or germanium. It is appreciated that protruding semiconductor fins 126′ and 226′ may also be formed of other materials.

Referring to process 304 as shown in FIG. 14 , the widths Wm1 and Wm2 (FIG. 11 ) of protruding semiconductor fins 126′ and 226′ are measured. The measurement may be performed using an optical measurement method, such as Critical Dimension Scanning Electron Microscope (CDSEM), Optical Critical Dimension (OCD) spectroscopy, or the like. In process 306 (FIG. 14 ), the measured widths Wm1 and Wm2 are compared with the target widths Wt1 and Wt2 of protruding semiconductor fins 126′ and 226′, respectively. The target widths Wt1 and Wt2 are the intended widths of protruding semiconductor fins 126′ and 226′, respectively. For example, if fin width difference (Wm1−Wt1) is equal to zero or is a negative value, protruding semiconductor fins 126′ are not to be thinned. If fin width difference (Wm2−Wt2) is equal to zero or is a negative value, protruding semiconductor fins 226′ are not to be thinned. Otherwise, if one or both of differences (Wm1−Wt1) and (Wm2−Wt2) is a positive value, the corresponding protruding semiconductor fins 126′ and 226′ are thinned, and process 308 in FIG. 14 is performed. In the illustrated example, it is assumed that both protruding semiconductor fins 126′ and 226′ need to be thinned. If one of protruding semiconductor fins 126′ and 226′ does not need to be thinned, the corresponding etching-recipe generation process, fin thinning process, post-thinning re-measurement process, and re-working process, etc., for the corresponding fins are skipped.

Based on the measurement results such as the differences (Wm1−Wt1) and (Wm2−Wt2), etching recipes for thinning semiconductor fins 126′ and 226′ are generated, as shown as process 308 in FIG. 14 . The etching recipes include, and are not limited to, the etching duration, the type of etching chemical, the temperature of the etching chemical and wafer 10, the concentration of the etching chemical (when wet etching is used), the flow rates and the pressure of the etching gas (when dry etching is used), etc. For example, if the difference (Wm1−Wt1) is high, higher temperatures, higher concentrations, longer etching duration, and/or the like may be adopted. Conversely, if the difference (Wm1−Wt1) is small, lower temperatures, lower concentrations, shorter etching duration, and/or the like may be adopted.

Referring to process 310 in FIG. 14 , protruding semiconductor fins 126′ (first-type fins) are thinned using an etching chemical that etches protruding semiconductor fins 126′, while the etching rate of protruding semiconductor fins 226′ is low. During the etching, both protruding semiconductor fins 126′ and 226′ (FIG. 11 ) are exposed to the etching chemical. Accordingly, a first etching selectivity, which is the ratio of the etching rate of protruding semiconductor fins 126′ to the etching rate of protruding semiconductor fins 226′, is desired to be as high as possible to keep the etching of semiconductor fins 226′ to be minimized. For example, the first etching selectivity may be higher than about 5, and may be in the range between about 5 and 20 (or higher).

In accordance with some embodiments in which protruding semiconductor fins 126′ are silicon fins, the etching may be performed using wet etching, and the etching chemical may include an organic or inorganic alkaline(s). For example, the etching chemical may include metal hydroxide (M^(n+)(OH⁻)_(n)), amine derivatives, or combinations thereof. The metal hydroxide may include NaOH, KOH, LiOH, RbOH, CsOH, or mixtures thereof. Ionic surfactants such as quaternary ammonium (—R₄N⁺), sulfate (—OSO₃ ⁻), sulfonate (—SO₃ ⁻), phosphate, carboxylates (—COO⁻) derivatives or nonionic surfactants such as alcohol ethoxylates, alkyl phenol ethoxylates, fatty acid ethoxylates, fatty amine ethoxylates, glycol esters, glycerol esters may be added to reduce the surface tension of the etching chemical. The amine derivatives may include Ammonia, Tetra Methyl Ammonium Hydroxide (TMAH), Tetra Ethyl Ammonium Hydroxide (TEAH), Tetradecyl Trimethyl Ammonium Hydroxide (TTAH), Tetra Butyl Ammonium Hydroxide (TBAH), or the like, or combinations thereof. FIG. 16 illustrates a chemical structure of an amine derivative, which includes an OH group, a nitrogen atom, and R1, R2, R3, and R4 bonded to the nitrogen atom. Each of R1, R2, R3, and R4 may be a hydrogen atom or one of functional groups C1 to C20. The functional group C1 to C20 may include amine, alcohol, halide, ester, ketone, acid, alkane, alkene, alkyne, ether, sulfide, aldehydes, imines, nitrile, aromatic, thiol, or the like. In the etching process, the temperature of the etching chemical may be in the range between about 5° C. and about 90° C.

Due to the high first etching selectivity, although protruding semiconductor fins 226′ are also exposed to the same etching chemical as protruding semiconductor fins 126′, protruding semiconductor fins 226′ is substantially not etched.

The protruding semiconductor fins 126′ and 226′ experienced the fin-thinning process are shown in FIG. 12 . After the thinning of protruding semiconductor fins 126′, the widths Wm1′ of protruding semiconductor fins 126′ are measured again, as shown as process 312 in FIG. 14 . The measured widths Wm1′ may then be compared with the target width Wt1 again. If fin width difference (Wm1′−Wt1) is a positive value, the process loops back to process 308 to perform a rework process, which includes processes 308 and 310. Accordingly, another etching recipe is generated based on the fin width difference (Wm1′−Wt1), and then protruding semiconductor fins 126′ are etched again using the newly generated etching recipe, which may be different from the previously generated etching recipe for the first etching of protruding semiconductor fins 126′. Otherwise, if fin width difference (Wm1′−Wt1) is equal to or smaller than 0, no rework will be performed, and process, instead of looping back to process 308, proceeds to the thinning of protruding semiconductor fins 226′.

As aforementioned, the etching recipe may include different process conditions. Furthermore, the generated etching recipes may include different concentrations of etching chemicals. Accordingly, when the concentration of the etching chemical is high, the etching rate of protruding semiconductor fins 126′ is high, and when the concentration of the etching chemical is low, the etching rate of protruding semiconductor fins 126′ is low. Different concentrations of the etching chemicals may be determined based on the fin-width differences (Wm1−Wt1) and (Wm1′−Wt1). For example, since the fin width difference (Wm1−Wt1) is greater than fin width difference (Wm1′−Wt1), the concentration of the etching chemical when protruding semiconductor fins 126′ are thinned first time may be higher than that in the rework process. FIG. 17 illustrates an apparatus for providing the etching chemicals and adjusting the concentrations of the etching chemical based on the generated recipes.

Referring to FIG. 17 , wafer 10 is provided for the fin-thinning, with etching chemical 340 being sprayed on the surface of wafer 10 from nozzle 348. Pipe 346 is connected between nozzle 348 and storages 342 and 344. Storage 342 is used for storing the etching chemical, while storage 344 is used for storing de-ionized water in accordance with some examples. Valve 350 is connected between storage 344 and pipe 346, and is configured to open, close, and adjust the flow of the de-ionized water. In the fin-thinning process, a control unit 338 (also shown in FIG. 14 ) controls the operation of valve 350 to add (or not to add) a desirable flow of the de-ionized water. The de-ionized water goes into pipe 346 and is mixed with the etching chemical from storage 342. The desirable flow rates of the etching chemical and the DI water are determined based on the etching recipe, and when the relative flow of the de-ionized water is greater, the resulting etching chemical sprayed from nozzle 348 is diluted more, and the concentration of the etching chemical is low, and vice versa.

After the etching and the possible reworking of protruding semiconductor fins 126′, protruding semiconductor fins 226′ are thinned if the measured width Wm2 (FIG. 11 ) indicate that it needs to be thinned. The resulting structure is shown in FIG. 13 . Otherwise, processes 314, 316, and 318 as shown in FIG. 14 are skipped, and the resulting fins in the final structure have the shape as shown in FIG. 12 . Process 314 in FIG. 14 illustrates the etching of protruding semiconductor fins 226′. The thinning is performed using an etching chemical that etches protruding semiconductor fins 226′, while the etching rate of protruding semiconductor fins 126′ is low. During the etching, both protruding semiconductor fins 126′ and 226 are exposed to the etching chemical. Accordingly, a second etching selectivity, which is the ratio of the etching rate of protruding semiconductor fins 226′ to the etching rate of protruding semiconductor fins 126′, is desired to be as high as possible to keep the etching of semiconductor fins 126′ to be minimized. For example, the second etching selectivity may be higher than about 5, and may be in the range between about 5 and 20 (or higher).

In accordance with some embodiments in which protruding semiconductor fins 226′ are silicon germanium fins or germanium fins, the etching may be performed using wet etching, and the etching chemical may include an organic or inorganic alkaline(s) and an oxidant(s). The organic or inorganic alkaline(s) may be the same as or different from the organic or inorganic alkaline(s) used in the thinning of protruding semiconductor fins 126′. For example, the etching chemical may include metal hydroxide (M^(n+)(OH⁻)_(n)), amine derivatives, or combinations thereof. The metal hydroxide may include NaOH, KOH, LiOH, RbOH, CsOH, or mixtures thereof. Ionic surfactants such as quaternary ammonium (—R₄N⁺), sulfate (—OSO₃ ⁻), sulfonate (—SO₃ ⁻), phosphate, carboxylates (—COO⁻) derivatives or nonionic surfactants such as alcohol ethoxylates, alkyl phenol ethoxylates, fatty acid ethoxylates, fatty amine ethoxylates, glycol esters, glycerol esters may be added to reduce the surface tension of the etching chemical. The amine derivatives may include Ammonia, TMAH, TEAH, TTAH, TBAH, or the like, or combinations thereof. The oxidant may include Dissolved ozone in DI water (DIO₃), hydrogen peroxide (H₂O₂), or combinations thereof. The etching chemical for etching protruding semiconductor fins 226′ does not substantially attack protruding semiconductor fins 126′. Accordingly, although protruding semiconductor fins 126′ is also exposed to the same etching chemical as protruding semiconductor fins 226′, protruding semiconductor fins 126′ is substantially not etched. In the etching, the temperature of the etching chemical may be in the range between about 5° C. and about 90° C.

The organic or inorganic alkaline(s) that are used for etching protruding semiconductor fins 126′ do not substantially attack protruding semiconductor fins 226′, as aforementioned. The oxidant is added to oxidize protruding semiconductor fins 226′, and the resulting silicon germanium oxide or germanium oxide can be etched by the aforementioned organic or inorganic alkaline(s). Accordingly, through the oxidation process, protruding semiconductor fins 226′ may also be thinned. On the other hand, the oxidation process also results in silicon oxide to be generated on protruding semiconductor fins 126′. The organic or inorganic alkaline(s) are not able to etch silicon oxide. Also, silicon is oxidized significantly slower than silicon germanium and germanium. Accordingly, protruding semiconductor fins 126′ is substantially un-thinned when protruding semiconductor fins 226′ are thinned.

In accordance with some embodiments, the organic or inorganic alkaline(s) and the oxidant are mixed in the same etching chemical to etch protruding semiconductor fins 226′. In accordance with other embodiments, the organic or inorganic alkaline(s) and the oxidant are in separate solutions, wherein the oxidant is used first to oxidize protruding semiconductor fins 126′ and 226′ to form oxide. The alkaline solution is then applied on wafer 10 to remove the generated oxide, followed by the removal of the alkaline solution. The cycle including the oxidant application and the alkaline application may or may not be repeated.

After the thinning of protruding semiconductor fins 226′, the widths Wm2′ of protruding semiconductor fins 226′ are measured again, as shown in process 316 in FIG. 14 . The measured widths Wm2′ is then compared with the target width Wt2 again. If fin width difference (Wm2′−Wt2) has a positive value, the process loops back to process 318 and then to process 316 to rework on the thinning of protruding semiconductor fins 226′. Accordingly, another etching recipe is generated (process 318) based on the fin width difference (Wm2′−Wt2), and then protruding semiconductor fins 226′ are thinned again using the newly generated etching recipe. Otherwise, if (Wm2′−Wt2) is equal to or smaller than 0, no rework will be performed.

In accordance with some embodiments of the present disclosure, instead of working on the measurement and the reworking of protruding semiconductor fins 126′ before the measurement and the thinning of protruding semiconductor fins 226′, the re-measurement of protruding semiconductor fins 126′ may be performed after the thinning of protruding semiconductor fins 226′. FIG. 15 illustrates a corresponding process flow 300′. Process flow 300′ is similar to process flow 300 in FIG. 14 , except that the re-measurement of fin widths and the re-work (if needed) is performed after the fins of both types (n-type and p-type) are thinned (process 310′). Correspondingly, the widths of both protruding semiconductor fins 126′ and 226′ are re-measured, and if one type or both of protruding semiconductor fins 126′ and 226′ need to be re-worked, the process goes back to process 308 for the re-work of the corresponding protruding semiconductor fins 126′ and 226′.

Referring to FIG. 13 , the structure in device region 100 is discussed as follows in accordance with some example embodiments. The structure in device region 200 may have essentially the same profile, and hence the profile is not discussed again. In accordance with some embodiments of the present disclosure, in the thinning of protruding semiconductor fins 126′ and 226′, STI regions 22 may be recessed slightly, and the sidewalls of the portions of the protruding semiconductor fins 126′ and 226′ in the recessed portions of STI regions 22 are also exposed. The newly exposed sidewalls of protruding semiconductor fins 126′ and 226′ are also etched, forming transition regions 126T. The sidewalls of transition regions 126T are less slanted than the sidewalls of upper portions of protruding semiconductor fins 126′ and the underlying semiconductor strips 126. In accordance with some embodiments, the slant angle θ1 of the sidewalls of the transition regions 126T is in the range between about 5° and about 85°. The transition angle θ2 is in the range between about 95° and about 175°. The sidewall SW1 of the top portion of a protruding fin 126′ may be in a first plane P1, which is parallel to a second plane P2 of the sidewall SW2 of semiconductor strip 126. Protruding semiconductor fins 126′ may be trimmed by amount ΔW on each side, with trimming amount ΔW being in the range between about 0.2 nm and about 30 nm. The height H3 of transition regions 126T may be equal to or greater than 0.5*ΔW and equal to or smaller than about 5*ΔW. After the fin-thinning, the fin widths Wm1′ may be in the range between about 2 nm and about 50 nm. The height H1 of STI regions 22 may be in the range between about 30 nm and about 100 nm. The fin height H2 may be in the range between about 5 nm and about 100 nm. The bottom width Wb of semiconductor strip 126 may be in the range between about 2.2 nm and about 80 nm.

In the fin-thinning process, control unit 338 (FIGS. 14, 15, and 17 ) is used to electrically and signally communicate with, and control, the various tools used in the fin-thinning process. The control unit 338 controls the actions involved in the fin-thinning process, which actions include, and are not limited to, the measurement and re-measurement of fin widths, the determination of whether the fin-thinning is needed, the generation of the etching recipes, and the fin-thinning processes. The control unit 338 and the tools in combination form an Advanced Process Control (APC) system for automatically measuring, thinning, and reworking on the fin-thinning.

FIGS. 19-21, 22, 23A, 23B, 24, 25A, 25B, 26-29, 30, 31A, and 31B illustrate the cross-sectional views and perspective views of intermediate stages in a fin-thinning process and the formation of FinFETs in device regions 100 and 200, respectively. The respective process flow is shown in the process flow 500 as shown in FIG. 34 . Unless specified otherwise, the materials, the structures, and the formation processes of the components in these embodiments are essentially the same as the like components denoted by like reference numerals in the preceding embodiments. The details regarding the materials, the structures, and the formation processes of the components shown in these embodiments may thus be found in the discussion of the preceding embodiments.

Referring to FIG. 19 , wafer 10 is formed. Wafer 10 includes device regions 100 and 200. The respective process is illustrated as process 502 in the process flow 500 as shown in FIG. 34 . In accordance with some embodiments, the device region 100 is an n-type FinFET region, in which an n-type FinFET is to be formed, and device region 200 is a p-type FinFET region, in which a p-type FinFET is to be formed. In accordance with other embodiments, device regions 100 and 200 may be both n-type FinFET regions, both p-type FinFET regions, or a p-type FinFET region and an n-type FinFET region, respectively.

Wafer 10 may include semiconductor substrate 20′, which may comprise silicon and/or other semiconductor materials. The portion of semiconductor substrate 20′ in device region 100 may include bulk semiconductor substrate 20, which may be formed of silicon. The portion of semiconductor substrate 20′ in device region 200 may include base semiconductor substrate 20 and an epitaxy semiconductor layer 21 over the bulk semiconductor substrate 20. In accordance with some embodiments, epitaxy semiconductor layer 21 includes germanium, and may include silicon germanium, or may include germanium without silicon. The germanium percentage in epitaxy semiconductor layer 21 may be higher than 20 percent, and may be in the range between about 20 percent and about 100 percent. In accordance with some embodiments, the formation of epitaxy semiconductor layer 21 may include etching bulk semiconductor substrate 20 to form a recess, and growing epitaxy semiconductor layer 21 through an epitaxy process. A planarization process such as a CMP process or a mechanical grinding process may then be performed to remove the portion of epitaxy semiconductor layer 21 from device region 100.

It is appreciated that the materials of silicon and silicon germanium in device regions 100 and 200, respectively, are an example, and different materials may be used in device regions 100 and 200. The usable materials may include silicon, germanium, a compound semiconductor including carbon-doped silicon, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide, an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP, combinations thereof, and/or multi-layers thereof.

Next, further referring to FIG. 19 , semiconductor buffer layers 166 and 266 are deposited. The respective process is illustrated as process 504 in the process flow 500 as shown in FIG. 34 . In accordance with some embodiments, semiconductor buffer layers 166 and 266 are deposited as silicon layers, and hence are alternatively referred to as silicon buffer layers hereinafter. In accordance with some embodiments, semiconductor buffer layers 166 and 266 are deposited in a same deposition process, or in separate deposition processes.

In accordance with some embodiments, the thickness of semiconductor buffer layers 166 and 266 is in the range between about 1 nm and about 20 nm, and may be in the range between about 2 nm and about 5 nm. The silicon atomic percentage in semiconductor buffer layers 166 and 266 may be greater than 90 percent, 95 percent, 99 percent, or higher.

In accordance with some embodiments, the deposition process includes an epitaxy process, so that the resulting semiconductor buffer layers 166 and 266 have a single crystalline structure. In accordance with alternative embodiments, semiconductor buffer layers 166 and 266 are deposited as having a polycrystalline structure or an amorphous structure. During the epitaxy, either one of semiconductor buffer layers 166 and 266 may be in-situ doped with a p-type or an n-type dopant. In accordance with alternative embodiments, semiconductor buffer layers 166 and 266 are intrinsic with no p-type and n-type dopant therein. Accordingly, due to the structure similarity or difference, and the dopant similarity or difference, semiconductor buffer layer 166 may be, or may not be, distinguishable from the underlying bulk semiconductor substrate 20. A dashed line is thus illustrated between semiconductor buffer layer 166 and the underlying bulk semiconductor substrate 20 to indicate that they may or may not be distinguishable. Semiconductor buffer layer 266 is distinguishable from the underlying epitaxy semiconductor layer 21 due to their difference in compositions.

In device region 100, semiconductor buffer layer 166 and bulk semiconductor substrate 20 are collectively referred to as substrate 20′. In device region 200, bulk semiconductor substrate 20, epitaxy semiconductor layer 21, and semiconductor buffer layer 266 are collectively referred to as substrate 20′. A p-well region (not shown) may be formed in semiconductor substrate 20′ and in device region 100, for example, through an ion implantation process to implant a p-type dopant. An n-well region (not shown) may be formed in semiconductor substrate 20′ and in device region 200, for example, through an ion implantation process to implant an n-type dopant. Accordingly, each of semiconductor buffer layers 166 and 266 may comprise, or may be free from, the dopant for forming the p-well region and the n-well region, respectively.

Referring to FIG. 20 , wafer 10 is etched to form trenches 23 and semiconductor strips 126 and 226. The respective process is illustrated as process 506 in the process flow 500 as shown in FIG. 34 . In device region 100, the portions of substrate 20′ between neighboring trenches 23 are referred to as semiconductor strips 126. In device region 200, the portions of substrate 20′ between neighboring trenches 23 are referred to as semiconductor strips 226. The trenches 23 are again formed by forming a blanket pad dielectric layer 28 and a blanket hard mask layer 29, and then etching the blanket hard mask layer 29 through an etching process. Next, the patterned hard mask layer 29 is used as an etching mask to etch pad dielectric layer 28 and the underlying substrate 20′. The resulting structure is shown in FIG. 20 . The resulting trenches 23 may penetrate through epitaxy semiconductor layer 21 to extend into the underlying bulk semiconductor substrate 20.

Referring to FIG. 21 , dielectric regions 22 are formed through deposition processes. The respective process is illustrated as process 508 in the process flow 500 as shown in FIG. 34 . Dielectric regions 22 are formed by filling trenches 23 (FIG. 20 ) with a dielectric material(s). In accordance with some embodiments, the formation of isolation regions 22 may include depositing a liner dielectric, and then depositing a dielectric material over the liner dielectric. The liner dielectric may include a thermal oxide layer formed through the thermal oxidation of a surface layer of the exposed semiconductor materials. The liner dielectric may also include a deposited silicon oxide layer, silicon nitride layer, and/or the like formed using, for example, ALD, HDPCVD, CVD, or the like. The dielectric material may be formed using FCVD, spin-on coating, or the like, and may include silicon oxide in accordance with some embodiments. Dielectric regions 22 have top surfaces higher than hard mask layer 29.

Referring to FIG. 22 , a planarization process is performed to remove excess portions of dielectric regions 22. The planarization process may include a CMP process, a mechanical grinding process, or the like. The remaining portions of dielectric regions 22 are referred to as STI regions 22. The respective process is illustrated as process 510 in the process flow 500 as shown in FIG. 34 .

In accordance with some embodiments, as shown in FIG. 22 , the planarization process is performed using semiconductor buffer layers 166 and 266 as CMP stop layers. Accordingly, semiconductor buffer layers 166 and 266 remain in the resulting structure, and are revealed. In accordance with alternative embodiments, pad dielectric layer 28 (FIG. 21 ) is used as the CMP stop layer. In accordance with yet alternative embodiments, hard mask layer 29 (FIG. 21 ) is used as the CMP stop layer.

Referring to FIGS. 23A, STI regions 22 are recessed, so that the top portions of semiconductor strips 126 and 226 protrude higher than the top surfaces 22A of the remaining portions of STI regions 22 to form protruding semiconductor fins 126′ and 226′, respectively. The respective process is illustrated as process 512 in the process flow 500 as shown in FIG. 34 . The portions of the semiconductor strips 126 and 226 lower than the top surfaces 22A remain to be referred to as semiconductor strips 126 and 226, respectively. The etching may be performed using a dry etching process, wherein HF and NH₃, for example, are used as the etching gases. During the etching process, plasma may be generated. In accordance with alternative embodiments of the present disclosure, the recessing of STI regions 22 is performed using a wet etching process. The etching chemical may include an HF solution, for example.

FIG. 23B illustrates the cross-sectional views of the structure shown in FIG. 23A. Again, device regions 100 and 200 are shown separately. The cross-sectional views in FIG. 23B are obtained from the vertical plane perpendicular to the lengthwise directions of protruding semiconductor fins 126′ and 226′, and illustrate the cross-sections A-A and B-B, respectively, in FIG. 23A. In accordance with some embodiments, the top surfaces 22A of the recessed STI regions 22 are higher than the remaining portions of epitaxy semiconductor layer 21.

In accordance with alternative embodiments, the top surfaces 22A of the recessed STI regions 22 are level with the remaining portions of epitaxy semiconductor layer 21. Due to the formation of semiconductor buffer layers 166 and 266, the top surfaces of protruding semiconductor fins 126′ and 226′ are coplanar with each other, and are at the same level, which is shown as level 67. Pad dielectric layers 28 and hard mask layers 29 may or may not exist, and are illustrated using dashed lines.

In accordance with some embodiments, after the recessing for STI regions 22 to form protruding semiconductor fins 126′ and 226′, a fin-thinning process may be performed to thin the protruding semiconductor fins 126′ and/or 226′ to improve the gate control, and to reduce the fin-width variation. The respective process is illustrated as process 514 in the process flow 500 as shown in FIG. 34 . The details of the fin-thinning process are the same as discussed in the preceding embodiments. For example, the details of the thinning processes are the same as discussed referring to FIGS. 11 through 18 . Accordingly, the details of the thinning processes are not repeated herein, and may be found referring to the discussion of FIGS. 11 through 18 .

FIG. 24 illustrates an example structure after the fin-thinning process, wherein both of protruding semiconductor fins 126′ and 226′ are thinned in accordance with the illustrated example embodiments. In accordance with some embodiments, when protruding semiconductor fins 126′ are thinned, semiconductor buffer layer 266 is also thinned since its material may be the same as or similar to the material of protruding semiconductor fin 126′.

In accordance with alternative embodiments, protruding semiconductor fins 126′ are not thinned, so that semiconductor buffer layer 266 is also not thinned. Accordingly, after protruding semiconductor fins 126′ are thinned, the edges of semiconductor buffer layer 266 may laterally extend beyond the respective edges of the underlying epitaxy semiconductor layer 21. This results in larger caps (formed of semiconductor buffer layers 266) being located above the narrower underlying protruding semiconductor fins 226′. For example, in the process 306 shown in FIG. 14 , it may be determined that protruding semiconductor fins 126′ do not need to be thinned, while protruding semiconductor fins 226′ are determined as needing thinning.

In accordance with some embodiments, after the fin-thinning processes, the top surfaces of semiconductor buffer layers 166 and 266 are exposed, and pad dielectric layer 28 and hard mask layer 29 have been removed in preceding processes. In accordance with alternative embodiments, pad dielectric layer 28 and hard mask layer 29 remain on top surfaces of semiconductor buffer layers 166 and 266. In accordance with these embodiments, etching processes may be performed to remove the remaining portions of oxide layer 28 and hard mask layer 29.

In accordance with some embodiments, protruding semiconductor fins 126′ and 226′ (especially 226′) are prone to wriggling problem when thinned. The wriggling is due to the strain in protruding semiconductor fins 226′ (and possibly 126′), and the wriggling may be related to the material difference between epitaxy layer 21 (FIG. 19 ) and the underling bulk semiconductor substrate 20. As shown in FIG. 32 , the strain may cause the top portions of semiconductor fin 226′ to bend. Furthermore, with the wriggling problem, different portions of the protruding fin 226′ bend differently, for example, to different directions (left and right directions in FIG. 32 ), and bend with different magnitude. This may cause the top portions of neighboring protruding semiconductor fins 226′ (possibly 126′) to have smaller spacings than the respective lower portions. The smaller spacings results in various problems such as the difficulty in the filling of the spacing with dummy gate stack, the difficulty in etching and removing materials from the spacings, etc.

With the formation of semiconductor buffer layers 166 and 266 on top of protruding semiconductor fins 126′ and 226′, respectively, semiconductor buffer layer 266 (and possibly 166 also) may apply strain to the underlying protruding semiconductor fin, and has the effect of limiting the wriggling of the underlying epitaxy semiconductor fin. The wriggling problem is thus reduced. Also, the effect of reducing the wriggling problem is related to the thickness of semiconductor buffer layers 166 and 266, with thicker semiconductor buffer layers 166 and 266 having greater effect in reducing the wriggling. Experiment results have revealed that the mean Line Edge Roughness (LER) of a thinned protruding semiconductor fins 226′ may be around 4.1 nm when no semiconductor buffer layer 266 is formed, wherein the LER is an indication of the magnitude of the wriggling. When semiconductor buffer layers 266 are formed as having a thickness of 2 nm, the mean LER is reduced to about 3.5 nm. When semiconductor buffer layers 266 are formed as having a thickness of 5 nm, the mean LER is further reduced to about 2.8 nm. When the thickness of semiconductor buffer layers 266 is further increased, the mean LER may be further reduced to as small as about 1.5 nm.

FIG. 32 schematically illustrates a sample protruding fin 226′ with wriggling problem, which sample protruding fin 226′ is thinned when no semiconductor buffer layer is over it to provide the strain. The top portions of the illustrated protruding fin 226′ bends toward left and right relative to the underlying portion. As a comparison, FIG. 33 illustrates a sample protruding fin 226′ with reduced or eliminated wriggling problem, which sample protruding fin 226′ is thinned when there is a semiconductor buffer layer (not shown, refer to FIG. 24 ) over it. FIG. 33 graphically illustrates that having a semiconductor buffer layer thereon during the thinning process may significantly reduce the fin wriggling problem.

In the lateral thinning of protruding semiconductor fins 126′ and 226′, protruding semiconductor fins 126′ and 226′ are also vertically thinned from top when semiconductor buffer layers 166 and 266 are the top layers (when pad dielectric layers 28 and hard mask layers 29 are not formed). As shown in FIG. 24 , by forming semiconductor buffer layers 166 and 266, which are formed of the same material, the vertical thinning rate of protruding semiconductor fins 126′ and 226′ are the same as each other. This ensures that the top ends of protruding semiconductor fins 126′ and 226′ are at the same level, which is marked as level 67′, which may be lower than level 67 in FIG. 23B. When pad dielectric layers 28 and hard mask layers 29 are over semiconductor buffer layers 166 and 266 during the thinning processes, the top ends of protruding semiconductor fins 126′ and 226′ are also at the same level 67′, which is the same as level 67 in FIG. 23B. Accordingly, by forming semiconductor buffer layers 166 and 266, and leaving semiconductor buffer layers 166 and 266 unremoved when protruding semiconductor fins 126′ and 226′ are thinned, the difference in the heights of protruding semiconductor fins 126′ and 226′ is reduced, and is controllable. This leads to the reduced performance variation.

FIGS. 25A and 25B illustrate the formation of silicon cap layers in accordance with some embodiments. The respective process is illustrated as process 516 in the process flow 500 as shown in FIG. 34 . FIG. 25B illustrates the cross-sections A-A and B-B obtained from cross-sections A-A and B-B, respectively, in FIG. 25A. Semiconductor cap layers 168 and 268 are selectively deposited on protruding fins 126′ and 226′, respectively, and are not deposited on dielectric materials such as STI regions 22. Semiconductor cap layers 168 and 268 may also be deposited in a same deposition process. In accordance with some embodiments, semiconductor cap layers 168 and 268 are formed as conformal layers, with the thicknesses of the horizontal portions being equal to or substantially equal to (for example, with the difference being smaller than about 10 percent) the thicknesses of the vertical portions. In accordance with alternative embodiments, the formation of semiconductor cap layers 168 and 268 is skipped, and no semiconductor cap layers are formed after the thinning process.

In accordance with some embodiments, the thicknesses BT1 and BT2 of semiconductor buffer layers 166 and 266, respectively, may be in the range between about 0.1 nm and about 20 nm. The heights STIH1 and STIH2 of STI regions 22 in device regions 100 and 200, respectively, may be in the range between about 30 nm and about 100 nm. Bottom widths BW1 and BW2 of protruding fins 126′ and 226′ may be smaller than the respective top widths TW1 and TW2, respectively, which top widths TW1 and TW2 may be in the range between about 2 nm and about 50 nm.

It is appreciated that directly over one semiconductor strip 126 or 226, there may be a single protruding fin 126′ or a single protruding fin 226′. Alternatively, over one semiconductor strip 126 or 226, there may be a plurality of protruding fins 126′ or 226′, with the neighboring protruding fins having pitches in the range between about 5 nm and about 1 μm. The fin heights FH1 and FH2 of protruding fins 126′ and 226′ may be in the range between about 5 nm and about 100 nm.

The deposition of semiconductor cap layers 168 and 268 may be performed using a conformal deposition method such as CVD. In accordance with some embodiments of the present disclosure, semiconductor cap layers 168 and 268 are formed of silicon, which may be free or substantially free from some other elements such as germanium, carbon, or the like. For example, the atomic percentage of silicon in semiconductor cap layers 168 and 268 may be higher than about 95 percent, 99 percent, or higher. In accordance with other embodiments, semiconductor cap layers 168 and 268 are formed of other semiconductor materials different from the materials of semiconductor strips 126 and/or 226. For example, semiconductor cap layers 168 and 268 may be formed of silicon germanium with a lower germanium concentration than that of epitaxy semiconductor layer 21. Semiconductor cap layers 168 and 268 may be epitaxially grown as crystalline semiconductor layers or may be formed as polycrystalline semiconductor layers, which may be achieved, for example, by adjusting the temperature and the growth rate in the deposition process.

Referring to FIG. 26 , dummy gate stacks 30 are formed to extend on the top surfaces and the sidewalls of (protruding) fins 126′ and 226′ and on semiconductor cap layers 168 and 268. The respective process is illustrated as process 518 in the process flow 500 as shown in FIG. 34 . Dummy gate stacks 30 may include dummy gate dielectrics 32 and dummy gate electrodes 34 over dummy gate dielectrics 32, and hard mask layers 36 over dummy gate electrodes 34. The formation process of dummy gate stacks 30 may be essentially the same as discussed in preceding embodiments.

Next, gate spacers 38 are formed on the sidewalls of dummy gate stacks 30. In accordance with some embodiments of the present disclosure, gate spacers 38 are formed of a dielectric material(s) such as silicon nitride, silicon carbo-nitride, or the like, and may have a single-layer structure or a multi-layer structure including a plurality of dielectric layers. Semiconductor cap layers 168 and 268 and semiconductor buffer layers 166 and 266 thus extend directly under and overlapped by each of dummy gates stacks 30 and spacers 38.

Next, recessing processes are performed to etch the portions of protruding semiconductor fins 126′ and 226′ (including buffer layers 166 and 266) and semiconductor cap layers 168 and 268. The etched portions are the portions not covered by dummy gate stacks 30 and gate spacers 38. The structure shown in FIG. 27 is thus formed. The respective process is illustrated as process 520 in the process flow 500 as shown in FIG. 34 . The recessing may be anisotropic, and hence the portions of protruding semiconductor fins 126′ and 226′ and semiconductor buffer layers 166 and 266 directly underlying dummy gate stacks 30 and gate spacers 38 are protected, and are not etched. The top surfaces of the recessed semiconductor strips/fins 126/126′ and 226/226′ may be lower than the top surfaces 22A of STI regions 22 in accordance with some embodiments. The spaces left by the etched portions of semiconductor fins/strips 126/126′ and 226/226′ and semiconductor buffer layers 166 and 266 are referred to as recesses 40, which comprise the portions located on the opposite sides of dummy gate stacks 30, and the portions between remaining portions of protruding semiconductor fins 126′ and 226′.

Next, epitaxy regions (source/drain regions) 142 and 242 are formed by selectively growing (through epitaxy) semiconductor materials starting from recesses 40, resulting in the structure in FIG. 28 . The details for forming source/drain regions 142 and 242 may be the same as discussed referring to FIG. 7 , and are not repeated herein. The respective process is illustrated as process 522 in the process flow 500 as shown in FIG. 34 .

FIG. 29 illustrates a perspective view of the structure after the formation of CESL 46 and ILD 48. The respective process is illustrated as process 524 in the process flow 500 as shown in FIG. 34 .

In a subsequent process, the dummy gate stacks 30 including hard mask layers 36, dummy gate electrodes 34, and dummy gate dielectrics 32 are etched, forming trenches 52 between gate spacers 38, as shown in FIG. 30 . The respective process is illustrated as process 526 in the process flow 500 as shown in FIG. 34 . The top surfaces and the sidewalls of protruding semiconductor fins 126′ and 226′ (or semiconductor cap layers 168 and 268, when formed) as shown in FIG. 25B are exposed to trenches 52.

In accordance with some embodiments, the fin-thinning process as discussed referring to FIGS. 23B and 24 may also be performed after trenches 52 (FIG. 30 ) are formed. The respective process is illustrated as process 528 in the process flow 500 as shown in FIG. 34 . The cross-sectional views in cross-sections A-A and B-B of in FIG. 30 will be the same as shown in FIGS. 23B and 24 . The thinning of protruding semiconductor fins 126′ and 226′ is performed through the trenches 52. Accordingly, the portions of protruding semiconductor fins 126′ and 226′ exposed to trenches 52 are thinned, while the portions of protruding semiconductor fins 126′ and 226′ directly under gate spacers 38 are not thinned. After the thinning process, semiconductor cap layers 168 and 268 may also be formed on the thinned protruding semiconductor fins 126′ and 226′. The cross-sectional view of the protruding semiconductor fins 126′ and 226′ and semiconductor cap layers 168 and 268 may be essentially the same as FIG. 25B. Alternatively, after the thinning process, no semiconductor cap layers are formed on the thinned protruding semiconductor fins 126′ and 226′. Performing the fin-thinning process during the step shown in FIG. 30 has the benefit of reducing the concern of fin wriggling since the length of the thinned portions are smaller. This is because the length of the thinned portions is limited by opposing gate spacers 38.

In accordance with some embodiments, the fin-thinning process as shown in FIG. 24 is performed, while the fin-thinning process at the step of FIG. 30 is not performed. In accordance with alternative embodiments, the fin-thinning process at the step of FIG. 30 is performed, while the fin-thinning process as shown in FIG. 24 is not performed. In accordance with yet alternative embodiments, the fin-thinning processes as shown in both of FIG. 24 and FIG. 30 are performed.

Next, as shown in FIGS. 31A and 31B, replacement gate stacks 160 and 260 are formed in trenches 52 (FIG. 30 ). The respective process is illustrated as process 530 in the process flow 500 as shown in FIG. 34 . Replacement gate stacks 160 include interfacial layers (ILs) 167 (FIG. 31B), and high-k dielectric layers 169 over ILs 167. Replacement gate stacks 260 include interfacial layers (ILs) 267, and high-k dielectric layers 269 over ILs 267. The formation process, the structures, and the materials of replacement gate stacks 160 and 260 have been discussed referring to preceding embodiments, and are not repeated herein. FIGS. 31A and 31B also illustrate the formation of hard masks 162 and 262. The respective process is illustrated as process 532 in the process flow 500 as shown in FIG. 34 . Transistors 164 and 264 are thus formed.

FIG. 31B illustrates the cross-sections C-C (in device region 100) and D-D (in device region 200) in accordance with some embodiments. As shown in device region 100, semiconductor buffer layer 166, which is a top portion of protruding semiconductor fin 166′, extends directly underlying gate spacers 38. Semiconductor buffer layer 166 does not extend on the sidewalls of protruding semiconductor fin 166′ , as may be found from FIG. 25B. Depending on the materials and structures, semiconductor buffer layer 166 may be or may not be distinguishable from the underlying portion of protruding fin 126′ and/or semiconductor cap layer 168 (FIG. 25B). For example, when Semiconductor buffer layer 166 is an amorphous or polysilicon layer or has a composition different from the underlying portion of protruding fin 126′ , it may be distinguished from the underlying crystalline portion of protruding fin 126′.

On the other hand, capping layer 168 may (or may not) be formed on the top and sidewalls of protruding semiconductor fin 166′. In accordance with some embodiments, semiconductor buffer layer 166 may further includes a portion directly underlying IL layer 167 (comprising silicon oxide, for example), which is a part of gate dielectric 156. The portion of semiconductor buffer layer 166 directly underlying IL layer 167 may be thinner than the portion of semiconductor buffer layer 166 directly underlying gate spacers 38 due to the cleaning and oxidation process in the formation of replacement gate stack 160.

As shown in device region 200, semiconductor buffer layer 266, which is a top portion of protruding semiconductor fin 266′, extends directly underlying gate spacers 38. Semiconductor buffer layer 266 does not extend on the sidewalls of protruding semiconductor fin 266′. On the other hand, capping layer 268 may (or may not) be formed on the top and sidewalls of protruding semiconductor fin 266′. Semiconductor buffer layer 266 may be distinguishable from the underlying portion of protruding fin 226′ and/or semiconductor cap layer 268 due to their difference in materials and/or structures. In accordance with some embodiments, semiconductor buffer layer 266 may further includes a portion directly underlying IL layer 267 (comprising silicon oxide, for example), which is a part of gate dielectric 256. The portion of semiconductor buffer layer 266 directly underlying IL layer 267 may be thinner than portion of semiconductor buffer layer 266 directly underlying gate spacers 38 due to the cleaning and oxidation process in the formation of replacement gate stack 260.

In accordance with some embodiments, in the regions directly underlying gate stacks 160 and 260, semiconductor buffer layers 166 and 266 may be kept on top of the underlying portion of protruding fins 126′ and 266′, respectively without being removed. For example, semiconductor cap layers 168 and 268 may protect semiconductor buffer layers 166 and 266 from being removed. Accordingly, the confining effect remains in the final structure. When semiconductor cap layers 168 and 268 are not formed, semiconductor buffer layers 166 and 266 may (or may not) be kept on tops of the underlying portion of protruding fins 126′ and 226′ without being removed.

In accordance with alternative embodiments, semiconductor buffer layer 266 is removed after the removal of dummy gate stacks 30 and before the formation of replacement gate stacks 160 and 260. When semiconductor buffer layer 266 is removed, the confinement effect also no longer exists. At this time, however, since the exposed portion of protruding fin 226′ is short, and gate spacers 38 confine the portions of the protruding fin 226′ on the opposing sides of the exposed portion of protruding fin 226′, the wriggling of protruding fin 226′ is also limited. In the resulting structure, semiconductor buffer layer 266 may have remaining portions directly under gate spacers 38, and does not have any portion directly underlying gate stack 260. Gate dielectric 256 may thus be in physical contact with the underlying portion of epitaxy layer 21 (FIG. 19 ), which may be formed of silicon germanium or germanium.

In accordance with alternative embodiments, semiconductor buffer layer 266 is not removed, and may be thinned during the formation of replacement gates. Accordingly, as shown in FIG. 31B, the portion of semiconductor buffer layer 266 directly under gate spacers 38 has thickness T1, and the portion of semiconductor buffer layer 266 directly under replacement gate stack 160 has thickness T2. Thickness T2 is smaller than thickness T1, and ratio T2/T1 may be smaller than about 50 percent in accordance with some embodiments.

The embodiments of the present disclosure have some advantageous features. By measuring the fin widths first, the etching recipe may be determined to ensure that the etched fins have their widths falling into desirable ranges. The etching recipes may be generated according to the measurement results to customize the etching for each wafer and each type of fins. The re-measurement and the re-work further improve the accuracy of the fin widths. The present application may be applied on single wafer fin-thinning process or a batch-type fin-process, in which selected sample wafers in a batch of wafers may be measured (rather than each of the wafers) to improve through-put. With the more accurate fin-width control, the gate control may be improved, current density may be improved, and threshold voltages may be better controlled.

Furthermore, by forming a semiconductor buffer layer over a semiconductor fin, and leaving the semiconductor buffer layer on top of the semiconductor fin when the semiconductor fin is thinned, the semiconductor buffer layer may provide a strain confining the thinned semiconductor fin, so that the wriggling problem of the thinned semiconductor fin is reduced.

In accordance with some embodiments of the present disclosure, a method includes forming isolation regions extending into a semiconductor substrate, wherein a first semiconductor strip is between the isolation regions; recessing the isolation regions, wherein a top portion of the first semiconductor strip protrudes higher than top surfaces of the isolation regions to form a first semiconductor fin; measuring a first fin width of the first semiconductor fin; generating a first etch recipe based on the first fin width; and performing a first thinning process on the first semiconductor fin using the first etching recipe. In an embodiment, the method further includes after the first thinning process, re-measuring a second fin width of the first semiconductor fin. In an embodiment, the method further includes, after the re-measuring the second fin width: generating a second etch recipe based on the second fin width; and performing a second thinning process on the first semiconductor fin using the second etching recipe.

In an embodiment, the method further includes measuring a third fin width of a second semiconductor fin, wherein the first semiconductor fin and the second semiconductor fin are formed of different materials; generating a third etch recipe based on the third fin width; and performing a third thinning process on the second semiconductor fin using the third etching recipe. In an embodiment, in the first thinning process, the second semiconductor fin is exposed to a first etching chemical used for the first thinning process, and is substantially un-thinned, and wherein in the third thinning process, the first semiconductor fin is exposed to a second etching chemical used for the third thinning process, and is substantially un-thinned. In an embodiment, the generating the first etch recipe comprises: determining a difference between the first fin width and a target fin width of the first semiconductor fin; and determining an etching time of the first thinning process based on the difference.

In an embodiment, the first thinning process is performed using wet etch. In an embodiment, the method further includes forming a dummy gate stack on the first semiconductor fin that has been thinned by the first thinning process. In an embodiment, the method further includes forming a dummy gate stack on the first semiconductor fin; forming gate spacers on opposite sides of the dummy gate stack; and removing the dummy gate stack to form a recess between the gate spacers, wherein the first thinning process is performed through the recess.

In accordance with some embodiments of the present disclosure, a method includes forming a first semiconductor fin protruding higher than first isolation regions on opposite sides of the first semiconductor fin, wherein the first semiconductor fin is formed of a first semiconductor material; forming a second semiconductor fin protruding higher than second isolation regions on opposite sides of the second semiconductor fin, wherein the second semiconductor fin is formed of a second semiconductor material different from the first semiconductor material; measuring a first fin width of the first semiconductor fin; measuring a second fin width of the second semiconductor fin; thinning the first semiconductor fin based on the first fin width using a first etching chemical, wherein the second semiconductor fin is exposed to the first etching chemical when the first semiconductor fin is thinned; and thinning the second semiconductor fin based on the second fin width.

In an embodiment, when the first semiconductor fin is thinned, the first semiconductor fin has a first etching rate, and the second semiconductor fin has a second etching rate smaller than the first etching rate. In an embodiment, the second semiconductor fin is thinned using a second etching chemical, and the first semiconductor fin is exposed to the second etching chemical when the second semiconductor fin is thinned. In an embodiment, when the second semiconductor fin is thinned, the first semiconductor fin has a third etching rate, and the second semiconductor fin has a fourth etching rate greater than the third etching rate. In an embodiment, the method further includes forming a first gate over the first semiconductor fin; forming first source/drain regions based on the first semiconductor fin and on opposite sides of the first gate; forming a second gate over the second semiconductor fin; and forming second source/drain regions based on the second semiconductor fin and on opposite sides of the second gate, wherein the first source/drain regions and the second source/drain regions are of opposite conductivity types.

In accordance with some embodiments of the present disclosure, a method includes forming a first semiconductor region, with sidewalls of the first semiconductor region being exposed; measuring a first width of the first semiconductor region; generating a first etching recipe based on the first width and a first target width of the first semiconductor region, wherein the first target width is an intended width of the first semiconductor region; and etching the first semiconductor region using the first etching recipe. In an embodiment, when the first semiconductor region is etched, the first semiconductor region protrudes above a top surface of a bulk semiconductor material under the first semiconductor region, and the top surface is exposed to an etching chemical used for etching the first semiconductor region.

In an embodiment, when the first semiconductor region is etched, the first semiconductor region protrudes above a top surface of an isolation region, and the top surface of the isolation region is exposed to an etchant used for etching the first semiconductor region. In an embodiment, when the first semiconductor region is etched, the first semiconductor region is in a trench between gate spacers. In an embodiment, the etching the first semiconductor region is performed using a wet etch process. In an embodiment, the first semiconductor region is etched using an etching chemical, and when the first semiconductor region is etched, sidewalls of a second semiconductor region is exposed to the etching chemical, and the method further comprises forming an n-type transistor based on the first semiconductor region; and forming a p-type transistor based on the second semiconductor region.

In accordance with some embodiments of the present disclosure, a method comprises depositing a first silicon layer over a first semiconductor region; forming dielectric isolation regions extending into the first silicon layer and the first semiconductor region; recessing the dielectric isolation regions, wherein a first portion of the first silicon layer and a second portion of the first semiconductor region are between the dielectric isolation regions, and protrude higher than top surfaces of the dielectric isolation regions to form a first semiconductor fin; thinning the first semiconductor fin, wherein after the first semiconductor fin is thinned, the first portion of the first silicon layer remains; and forming a gate stack on the first semiconductor fin.

In an embodiment, the first semiconductor region comprises silicon germanium. In an embodiment, the forming the dielectric isolation regions comprises etching the first silicon layer and the first semiconductor region to form trenches; depositing a dielectric material to fill the trenches; and performing a planarization process using the first silicon layer as a planarization stop layer. In an embodiment, the forming the dielectric isolation regions comprises forming a patterned hard mask over the first silicon layer; etching the first silicon layer and the first semiconductor region to form trenches; depositing a dielectric material to fill the trenches; and performing a planarization process using the patterned hard mask as a planarization stop layer.

In an embodiment, the depositing the first silicon layer comprises an epitaxy process. In an embodiment, the first silicon layer is deposited as a polysilicon layer. In an embodiment, the first silicon layer is deposited as an amorphous silicon layer. In an embodiment, the gate stack is a replacement gate stack, and the method further comprises forming a dummy gate stack on a first part of the first semiconductor fin; forming a source/drain region based on a second part of the first semiconductor fin; and before the replacement gate stack is formed, removing the dummy gate stack, wherein after the removing, the first silicon layer remains.

In an embodiment, the method further comprises depositing a second silicon layer over a second semiconductor region, wherein the first semiconductor region and the second semiconductor region are formed of different materials, wherein the dielectric isolation regions further extend into the second silicon layer and the second semiconductor region, and wherein a third portion of the second silicon layer and a fourth portion of the second semiconductor region form a second semiconductor fin; thinning the second semiconductor fin; and forming an additional gate stack on the second semiconductor fin. In an embodiment, when the first semiconductor fin is thinned by a chemical, the second semiconductor fin is exposed to the chemical, and wherein the second semiconductor fin is substantially un-thinned when the first semiconductor fin is thinned.

In accordance with some embodiments of the present disclosure, a method comprises forming a semiconductor strip between opposing trenches, wherein the semiconductor strip comprises a first semiconductor layer; and a second semiconductor layer over the first semiconductor layer, wherein the first semiconductor layer and the second semiconductor layer comprise different semiconductor materials; filling the opposing trenches with a dielectric material; planarizing the dielectric material to form dielectric regions, wherein after the planarizing, a portion of the second semiconductor layer remains; and laterally thinning the semiconductor strip, wherein when the laterally thinning is performed, the second semiconductor layer is over the first semiconductor layer.

In an embodiment, the method further comprises forming a dummy gate stack on the semiconductor strip, wherein the semiconductor strip is thinned before the dummy gate stack is formed; forming source/drain regions on opposing sides of the dummy gate stack; removing the dummy gate stack to reveal the semiconductor strip; and forming a replacement gate stack on the semiconductor strip. In an embodiment, the method further comprises forming a dummy gate stack on the semiconductor strip; forming source/drain regions on opposing sides of the dummy gate stack; removing the dummy gate stack to reveal the semiconductor strip, wherein the semiconductor strip is thinned after the dummy gate stack is removed; and forming a replacement gate stack on the semiconductor strip.

In an embodiment, the planarizing the dielectric material is performed using the second semiconductor layer as a planarization stop layer. In an embodiment, the method further comprises laterally thinning an additional semiconductor strip using a chemical, wherein when the additional semiconductor strip is laterally thinned, the semiconductor strip is exposed to the chemical, and wherein the additional semiconductor strip is thinned at a higher etching rate than the semiconductor strip. In an embodiment, the method further comprises, after the semiconductor strip is laterally thinned, depositing a silicon layer on a top surface and a sidewall of the semiconductor strip, wherein the silicon layer covers the second semiconductor layer.

In accordance with some embodiments of the present disclosure, a method comprises depositing a silicon layer over a first semiconductor region and a second semiconductor region that are formed of different semiconductor materials; performing a patterning process to form a first semiconductor strip comprising first remaining portions of the first semiconductor region and the silicon layer; and a second semiconductor strip comprising second remaining portions of the second semiconductor region and the silicon layer; forming a dielectric material comprising parts on opposing sides of the first semiconductor strip and the second semiconductor strip, wherein the dielectric material comprises an upper portion over the first semiconductor strip and the second semiconductor strip; performing a planarization process to remove the upper portion of the dielectric material and to form isolation regions; recessing the isolation regions, so that top portions of the first semiconductor strip and the second semiconductor strip protrude higher than top surfaces of the isolation regions to form a first semiconductor fin and a second semiconductor fin, respectively; and thinning at least one of the first semiconductor fin and the second semiconductor fin.

In an embodiment, the second semiconductor region comprises silicon germanium, and the second semiconductor fin is thinned, and wherein after the second semiconductor fin is thinned, a portion of the silicon layer remains as a top portion of the second semiconductor fin. In an embodiment, when the second semiconductor fin is thinned, the portion of the silicon layer remaining as a top portion of the second semiconductor fin is substantially un-thinned. In an embodiment, the method further comprises, after the thinning, depositing a silicon cap layer on both of the first semiconductor fin and the second semiconductor fin; and forming gate stacks over the silicon cap layer and the first semiconductor fin and the second semiconductor fin.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method comprising: depositing a first silicon layer over a first semiconductor region; forming dielectric isolation regions extending into the first silicon layer and the first semiconductor region; recessing the dielectric isolation regions, wherein a first portion of the first silicon layer and a second portion of the first semiconductor region are between the dielectric isolation regions, and protrude higher than top surfaces of the dielectric isolation regions to form a first semiconductor fin; thinning the first semiconductor fin, wherein after the first semiconductor fin is thinned, the first portion of the first silicon layer remains; and forming a gate stack on the first semiconductor fin.
 2. The method of claim 1, wherein the first semiconductor region comprises silicon germanium.
 3. The method of claim 1, wherein the forming the dielectric isolation regions comprises: etching the first silicon layer and the first semiconductor region to form trenches; depositing a dielectric material to fill the trenches; and performing a planarization process using the first silicon layer as a planarization stop layer.
 4. The method of claim 1, wherein the forming the dielectric isolation regions comprises: forming a patterned hard mask over the first silicon layer; etching the first silicon layer and the first semiconductor region to form trenches; depositing a dielectric material to fill the trenches; and performing a planarization process using the patterned hard mask as a planarization stop layer.
 5. The method of claim 1, wherein the depositing the first silicon layer comprises an epitaxy process.
 6. The method of claim 1, wherein the first silicon layer is deposited as a polysilicon layer.
 7. The method of claim 1, wherein the first silicon layer is deposited as an amorphous silicon layer.
 8. The method of claim 1, wherein the gate stack is a replacement gate stack, and the method further comprises: forming a dummy gate stack on a first part of the first semiconductor fin; forming a source/drain region based on a second part of the first semiconductor fin; and before the replacement gate stack is formed, removing the dummy gate stack, wherein after the removing, the first silicon layer remains.
 9. The method of claim 1 further comprising: depositing a second silicon layer over a second semiconductor region, wherein the first semiconductor region and the second semiconductor region are formed of different materials, wherein the dielectric isolation regions further extend into the second silicon layer and the second semiconductor region, and wherein a third portion of the second silicon layer and a fourth portion of the second semiconductor region form a second semiconductor fin; thinning the second semiconductor fin; and forming an additional gate stack on the second semiconductor fin.
 10. The method of claim 9, wherein when the first semiconductor fin is thinned by a chemical, the second semiconductor fin is exposed to the chemical, and wherein the second semiconductor fin is substantially un-thinned when the first semiconductor fin is thinned.
 11. A method comprising: forming a semiconductor strip between opposing trenches, wherein the semiconductor strip comprises: a first semiconductor layer; and a second semiconductor layer over the first semiconductor layer, wherein the first semiconductor layer and the second semiconductor layer comprise different semiconductor materials; filling the opposing trenches with a dielectric material; planarizing the dielectric material to form dielectric regions, wherein after the planarizing, a portion of the second semiconductor layer remains; and laterally thinning the semiconductor strip, wherein when the laterally thinning is performed, the second semiconductor layer is over the first semiconductor layer.
 12. The method of claim 11 further comprising: forming a dummy gate stack on the semiconductor strip, wherein the semiconductor strip is thinned before the dummy gate stack is formed; forming source/drain regions on opposing sides of the dummy gate stack; removing the dummy gate stack to reveal the semiconductor strip; and forming a replacement gate stack on the semiconductor strip.
 13. The method of claim 11 further comprising: forming a dummy gate stack on the semiconductor strip; forming source/drain regions on opposing sides of the dummy gate stack; removing the dummy gate stack to reveal the semiconductor strip, wherein the semiconductor strip is thinned after the dummy gate stack is removed; and forming a replacement gate stack on the semiconductor strip.
 14. The method of claim 11, wherein the planarizing the dielectric material is performed using the second semiconductor layer as a planarization stop layer.
 15. The method of claim 11 further comprising laterally thinning an additional semiconductor strip using a chemical, wherein when the additional semiconductor strip is laterally thinned, the semiconductor strip is exposed to the chemical, and wherein the additional semiconductor strip is thinned at a higher etching rate than the semiconductor strip.
 16. The method of claim 11 further comprising, after the semiconductor strip is laterally thinned, depositing a silicon layer on a top surface and a sidewall of the semiconductor strip, wherein the silicon layer covers the second semiconductor layer.
 17. A method comprising: depositing a silicon layer over a first semiconductor region and a second semiconductor region that are formed of different semiconductor materials; performing a patterning process to form: a first semiconductor strip comprising first remaining portions of the first semiconductor region and the silicon layer; and a second semiconductor strip comprising second remaining portions of the second semiconductor region and the silicon layer; forming a dielectric material comprising parts on opposing sides of the first semiconductor strip and the second semiconductor strip, wherein the dielectric material comprises an upper portion over the first semiconductor strip and the second semiconductor strip; performing a planarization process to remove the upper portion of the dielectric material and to form isolation regions; recessing the isolation regions, so that top portions of the first semiconductor strip and the second semiconductor strip protrude higher than top surfaces of the isolation regions to form a first semiconductor fin and a second semiconductor fin, respectively; and thinning at least one of the first semiconductor fin and the second semiconductor fin.
 18. The method of claim 17, wherein the second semiconductor region comprises silicon germanium, and the second semiconductor fin is thinned, and wherein after the second semiconductor fin is thinned, a portion of the silicon layer remains as a top portion of the second semiconductor fin.
 19. The method of claim 18, wherein when the second semiconductor fin is thinned, the portion of the silicon layer remaining as a top portion of the second semiconductor fin is substantially un-thinned.
 20. The method of claim 17 further comprising: after the thinning, depositing a silicon cap layer on both of the first semiconductor fin and the second semiconductor fin; and forming gate stacks over the silicon cap layer and the first semiconductor fin and the second semiconductor fin. 